1. Field of the Invention
The present invention relates to software state transitions and, in particular, to execution of transition and entry code upon such transitions.
2. Description of the Related Art
Computer programs (software) are typically developed by programmers (program or code developers) writing the code for the program. The code is often written in a high-level language such as C++, Pascal, Fortran, Basic, or Forth. Software developers typically utilize a variety of techniques and tools in developing code, such as a textual programming environment (comprising editors, compilers, source-level debuggers, source management systems, and the like) and a real-time operating system (OS).
Graphical programming tools are also used to generate code. Such tools often provide an object-oriented programming environment, which allows code to be written as a collection of concurrent software objects or actors that communicate by messages. Each actor is a state machine that reacts based on messages received. An actor reacts to a message by transitioning from one state to another state via a state transition, or by executing code and remaining in the same state (a self-transition). An actor or state machine is always xe2x80x9cinxe2x80x9d a state, as the state transitions are considered to be instantaneous. An actor is in a particular state when there has been a state transition to the state and no transition leaving that state has occurred yet. Actors wait for messages from other actors, and then change states or take other action based on the message.
Both state transitions and self-transitions can cause state transition code to be executed. The way an actor reactsxe2x80x94which transitions and associated transition code is executedxe2x80x94based on a message may be referred to as its behavior. An actor can send messages to itself or to any other actor. Each state of a given state machine or actor can have sub-states, which are themselves states and may also have sub-states.
During coding, the object-oriented code development tool displays, for a given actor, the defined states and state transitions graphically to aid programming. The real-time object-oriented modeling (ROOM) is one object-oriented software development system which is often used as the basis for actual code development tools. In addition to graphical techniques, ROOM uses traditional programming languages to express detailed structure and behavior. For example, C or C++ describes actions to be taken on state transitions or state entry/exit. Further information on ROOM may be found in B. Selic, G. Gullekson, and Paul T. Ward, Real-Time Object-Oriented Modeling (John Wiley and Sons, 1994). One code development tool which is based on the ROOM system is ObjecTime(copyright) code development software  less than www.objectime.com greater than . The output of running a ROOM-type code development tool such as ObjecTime(copyright) software is non-executable code written in a high-level language such as C++. A C++ compiler, in conjunction with ObjecTime(copyright) software, is therefore needed to compile the generated code into an executable application.
The resulting code generated by such a tool, for a given actor, can therefore be represented by a state diagram having a plurality of states and sub-states, interconnected by a series of state transitions. Referring now to FIG. 1, there is shown a state transition diagram 100 illustrating state transitions and self-transitions in an object-oriented software application developed with an object-oriented code development tool or program such as ObjecTime. State diagram 100 shows three states A, B, and C, where state A comprises sub-states A1 and A2. A state transition is made from state B to sub-state A1, via state transitions 111 and 111xe2x80x2. Similarly, a state transition is made from state C to sub-state A1, by transitions 112 and 112xe2x80x2.
Unlike a state transition, which is from one state to another, a self-transition (e.g. 113) is a transition that begins and ends on the same state. Both a self-transition and a state-change type of transition can cause transition code to be executed during the transition. As mentioned above, this transition code may be coded in a language such as C or C++.
The interaction of several features of typical object-oriented code development languages can give rise to unexpected and undesirable problems. In particular, the interaction of the self-transition, transition-to-history, and entry code features can cause entry code to be erroneously executed due to a self-transition.
Entry code is code associated with a given state that is fired (i.e., executed) every time the state is reached (entered). Similarly, exit code is fired every time a state is left (exited). Entry code is useful where, for example, there are many different state transitions into a state which would cause the same transition code to be executed. In this case, it may be more efficient to put this code in entry code for the state, rather than on all state transitions to the state, so that the code for the behavior has to be written only once, not once for each transition to the state.
For example, referring once more to FIG. 1, sub-state A1 may be a xe2x80x9cfaultxe2x80x9d state reached only when a problem has been detected. The entry code for sub-state A1 may turn on an xe2x80x9cemergencyxe2x80x9d light, while the exit code may turn off the light. The entry code for state A itself may be set to xe2x80x9cnullxe2x80x9d if there is no need for entry code for state A. Entry code is xe2x80x9cset to nullxe2x80x9d by simply coding no entry code for a given state, i.e. it does not exist for a given state. Thus, when states B or C make a state transition to sub-state A1, the code for turning on the emergency light does not need to be duplicated as part of the transition code for each state transition 111xe2x80x2, 112xe2x80x2, but is only coded once in the entry code for sub-state A1.
As noted above, a self-transition is a transition that begins and ends on the same state. Some object-oriented code development tools offer a transition to history feature. In the transition to history feature, when there is a transition to a state, the state automatically enters the most previously-entered sub-state, unless the state transition specifies otherwise. When a state is reached from an outside state by a state transition for the first time (e.g., the xe2x80x9cfirst timexe2x80x9d since initialization or beginning the program or the actor having the state), it has not yet been in a previous sub-state. In this case, no sub-state of the state is automatically entered and the transition to history feature has no effect.
State transitions to a state cause entry code for the state to fire, and, due to the transition to history feature, also cause the entry code for the sub-state to fire. Some self-transitions can also cause entry code to fire, including entry code for a sub-state automatically re-entered as a result of the transition to history feature. However, it is not always desired that entry code fire on self-transition. It is often desirable that entry code fire only when the state or sub-state is entered from a state external to the state. Accordingly, use of both self-transitions/transitions to history and entry code can result in unexpected and undesirable program behavior.
As an example, the state transition diagram 100 of FIG. 1 illustrates a variety of state transitions and self-transitions, and states and sub-states, of a given actor of an applications program. The program may comprise thousands of actors, each characterized by its own state diagram. Suppose state A is in sub-state A1 and is then left, e.g. by a transition to state B. Later, if there is a transition back to state A, the transition to history feature causes the most recent sub-state A1 to be automatically re-entered. This automatic state transition is indicated by the dotted arrow connected to state transition 115, and happens unless the state transition to state A specifies otherwise. When state A1 is entered in this manner, due to transition to history and a self-transition at a higher level, its entry code is fired.
Thus, when a self-transition at a higher level (e.g., state A) occurs, the transition to history feature causes the current sub-state A1 to be entered again as soon as the self-transition at state A occurs. This causes the entry code for both state A and sub-state A1 to be fired each time there is a state A self-transition when state A is in sub-state A1. In this example, when self-transition 113 occurs, the following code is executed in the order shown: sub-state A1 exit code; state A exit code; self-transition 113 transition code; state A entry code; sub-state A1 entry code.
Thus, entry code for a state or sub-state will be fired whenever the state is reached, whether the state is reached from outside by a state transition or due to a self-transition. However, it is not always desired that entry code fire on self-transition. For example, the entry code for a given state such as sub-state A1 may initialize a variable that increments so long as the state is not exited. If there is a self-transition, this can also fire the entry code and re-initialize the variable, even though the variable should not have been re-initialized.
A program using entry code, therefore, may produce unintended or unexpected results when a state""s entry code is fired not only when it is reached by an outside transition but also when a self-transition occurs. The code developer may unintentionally introduce such a entry code/self-transition conflict. This problem is exacerbated, due to the transition to history feature, when there are self-transitions at a higher levels, because it is not as easy to recognize such potential problems when there are many hierarchical levels of states and sub-states, and many state transitions and self-transitions. The potential problems and undesirable effects arising from the interaction between self-transitions and entry code can lead to code developers sacrificing one of these features in order to use the other.
A computer system runs an applications program generated by an object-oriented software development program. A state transition is made to a state of the applications program from an outside state of the applications program. All state transitions to the state from outside states are connected to an input of a choice point of the state. A choice transition of the choice point is selected in response to the state transition and the choice transition transitions back to the state, wherein the choice transition contains choice transition code which fires during the choice transition.